The present invention relates to a paint composition excellent in weatherability and coating gloss.
Recently, from the standpoint of elimination of the need of maintenance, there has been an increasing demand for highly durable materials, and it has been required that paint resins also possess higher weatherability. As of now, fluororesins comprised of a fluoroolefin-vinyl ether-series copolymer, acrylic silicon resins, and others have been known as paint resins with excellent weatherability. However, the fluororesins have problems that they are insufficient in recoatability and coating gloss, and when used as thermosetting paints smoothness of the resulting coatings is unsatisfactory, and the resulting coatings turn yellow. Moreover, acrylic silicone resins require humidity at their curing and therefore there arise the problems that airing in winter may result in inadequate curing and that they are not cured only by application of heat. In addition, acrylic silicone resins are still insufficient in weatherability and their coating gloss and adhesion to substrates are also below satisfactory levels. For solving such problems, epoxy group-containing (meth)acrylic copolymers, which are heat-set using a dibasic acid or an acid group-containing resin as a curing agent, have been in wide use as powder paints or acid rain-resistant paints, yet these paints are also unsatisfactory in weatherability.
It is an object of the present invention to provide a paint composition which is curable at ordinary temperature or under application of heat, unsusceptible to external environment upon curing, excellent in coating gloss, and capable of providing a coating with high weatherability, and which is further excellent in adhesion and does not turn yellow.
The inventors of the present invention have made intensive studies to solve the aforementioned problems and finally found that the problem is solved by a paint composition containing a copolymer which comprises, as constitutional units,
(A) an acrylic or methacrylic ester monomer unit having a straight-chain, branched-chain, or cyclic alkyl group;
(B) at least one monomer unit selected from the group consisting of a monomer unit having an unsaturated double bond and a hydroxyalkyl group, a monomer unit having an unsaturated double bond and an epoxy group, and a monomer unit having an unsaturated double bond and an alkoxysilyl group; and
(C) a monomer unit having an unsaturated double bond and a maleimide group.
That is, the present invention is a paint composition composed of a (meth)acrylic copolymer containing the aforementioned constitutional units (A), (B), and (C) as essential components. In this specification, xe2x80x9c(meth)acrylicxe2x80x9d means xe2x80x9cacrylic or methacrylicxe2x80x9d, and xe2x80x9c(meth)acrylatexe2x80x9d means xe2x80x9cacrylate or methacrylatexe2x80x9d.
Hereinafter, the present invention will be described in further detail.
The (meth)acrylic ester monomer as the constitutional unit (A) in the present invention has a straight-chain, branched-chain, or cyclic alkyl group, and the alkyl group may contain a phenyl group, an oxygen atom, a nitrogen atom, a sulfur atom, a halogen atom, and others.
Examples of the (meth)acrylic ester having a straight-chain alkyl group include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, octyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate, cetyl (meth)acrylate, stearyl (meth)acrylate, and behenyl (meth)acrylate. The monomer may be one having an aromatic ring, an oxygen atom, a nitrogen atom, a sulfur atom, a halogen atom, or others in its alkyl group, examples of which include benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, methoxyethyl (meth)acrylate, ethylcarbitol (meth)acrylate, ethoxypolyethylene glycol 400 (meth)acrylate, aminoethyl (meth)acrylate, chloroethyl (meth)acrylate, trifluoroethyl (meth)acrylate, and heptadecafluorooctylethyl (meth)acrylate.
Examples of the (meth)acrylic ester having a branched-chain alkyl group include isopropyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, neopentyl (meth)acrylate, ethylhexyl (meth)acrylate, isodecyl (meth)acrylate, and isostearyl (meth)acrylate. The monomer may be one having an aromatic ring, an oxygen atom, a nitrogen atom, a sulfur atom, a halogen atom, or others in its alkyl group, examples of which include 3-methoxybutyl (meth)acrylate, methoxytripropylene glycol (meth)acrylate, and hexafluoropropyl (meth)acrylate.
Examples of the (meth)acrylic ester having a cyclic alkyl group include cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, and tricyclodecynyl (meth)acrylate. The monomer may be one having an aromatic ring, an oxygen atom, a nitrogen atom, a sulfur atom, a halogen atom, or others in its alkyl group, examples of which include tetrahydrofurfuryl (meth)acrylate and morpholinoethyl (meth)acrylate.
These (meth)acrylic esters can be used singly or in combination.
Among the (meth)acrylic esters exemplified above, from the viewpoint of hardness of coatings and weatherability, those having an alkyl group of 1 to 20 carbon atoms are preferred, and those having an alkyl group of 1 to 8 carbon atoms are particularly preferred. Examples of such monomers include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, ethylhexyl (meth)acrylate, and cyclohexyl (meth)acrylate.
When hardness of coatings is of particular importance, monomers having an alkyl group of 1 to 7 carbon atoms are preferred. Examples of such monomers include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, and cyclohexyl (meth)acrylate.
If prevention of lifting phenomenon described later is of particular importance, it is preferred that a monomer having an alkyl group of 8 to 20 carbon atoms is used. Examples of such monomer include octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate, cetyl (meth)acrylate, palmitoyl (meth)acrylate, stearyl (meth)acrylate, behenyl (meth)acrylate, octadecanoyl (meth)acrylate, oleyl (meth)acrylate, icosanoyl (meth)acrylate, and isobornyl (meth)acrylate.
The constitutional unit (B) in the present invention can be selected from the group consisting of a monomer unit having an unsaturated double bond and a hydroxyalkyl group, a monomer unit having an unsaturated double bond and an epoxy group, and a monomer unit having an unsaturated double bond and an alkoxysilyl group.
Examples of the monomer having an unsaturated double bond and a hydroxyalkyl group are hydroxyalkyl group-containing ethylenically unsaturated monomers including hydroxyl group-containing monomers such as hydroxybutyl vinyl ether, hydroxyethyl allyl ether, and hydroxyethyl crotonate besides (meth)acrylic esters such as hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, 2-hydroxychloropropyl (meth)acrylate and commercial products available under the tradename xe2x80x9cPlaccel Fxe2x80x9d product family (i.e., lactone-modified hydroxyethyl-modified (meth)acrylic ester) manufactured by Daicel Chemical Industries, Ltd. Of these, monomers having a primary hydroxyl group are preferred because they easily react with curing agents such as isocyanates and melamine. Examples of such monomers include hydroxyethyl (meth)acrylate, hydroxybutyl (meth)acrylate, and the aforementioned commercial products xe2x80x9cPlaccel Fxe2x80x9d.
Examples of the monomer having an unsaturated double bond and an epoxy group are epoxy group-containing ethylenically unsaturated monomers including glycidyl (meth)acrylate, single terminal (meth)acrylic acid adducts of bisphenol A diglycidyl ether, allyl glycidyl ether, and glycidyl vinyl ether. Of these, glycidyl (meth)acrylate is preferred due to its copolymerizability with other monomers. This monomer unit is particularly useful in producing a thermosetting paint. The thermosetting paint is a paint which, after a main agent of the paint and a curing agent have been mixed together, is crosslinked and forms a tough coating upon heating, and is usually used in the form of a solvent- or powder-type paint.
The monomer having an unsaturated double bond and an alkoxysilyl group is a compound represented by the following formula (5):
Rxe2x80x94SiXnY3-nxe2x80x83xe2x80x83(5)
wherein R is a group having an olefinic unsaturated double bond; X is an alkyl group having 1 to 20 carbon atoms; Y is a hydrolyzable group; and when there are a plurality of Xs or Ys, the Xs and the Ys may independently be the same or different; and n is 0, 1, or 2.
Specific examples of the group denoted by R in the formula (5) include vinyl, allyl, butenyl, vinyloxy, allyloxy, acryloyl, methacryloyl, CH2xe2x95x90CHO(CH2)3xe2x80x94, CH2xe2x95x90CHCOO(CH2)3xe2x80x94, CH2xe2x95x90CHOCO(CH2)3xe2x80x94, CH2xe2x95x90C(CH3)COO(CH2)3xe2x80x94, and CH2xe2x95x90C(CH3)COO(CH2)2xe2x80x94Oxe2x80x94(CH2)3xe2x80x94 groups. Among these, CH2xe2x95x90CHCOO(CH2)3xe2x80x94, CH2xe2x95x90C(CH3)COO(CH2)3xe2x80x94, and vinyl groups are preferred.
Specific examples of the group denoted by X include methyl, ethyl, propyl, octyl, and octadecyl groups. Specific examples of the group denoted by Y is an alkoxy group. Concrete examples of the alkoxy group are methoxy, ethoxy, propoxy, butoxy, and methoxyethoxy groups, and methoxy and ethoxy groups particularly preferred.
Examples of the monomer having an unsaturated double bond and an alkoxysilyl group include CH2xe2x95x90C(CH3)COO(CH2)3Si(OCH3)3, CH2xe2x95x90C(CH3)COO(CH2)3Si(OC2H5)3, CH2xe2x95x90C(CH3)COO(CH2)3SiCH3(OCH3)2, CH2xe2x95x90CHCOO(CH2)3Si(OCH3)3, CH2xe2x95x90CHCOO(CH2)3Si(CH3)2(OC2H5), CH2xe2x95x90CHSi(OCH3)3, and CH2xe2x95x90CHSi(OC2H5)3.
The constitutional unit (C) in the present invention is a monomer having an unsaturated double bond and a maleimide group, and it is preferred that the monomer is an ethylenically unsaturated monomer having a maleimide group represented by the following formula (1): 
wherein R1 and R2 are independently a hydrogen atom or an alkyl group having 4 or less carbon atoms; one of R1 and R2 is a hydrogen atom and the other is an alkyl group having 4 or less carbon atoms; or R1 and R2 are a hydrocarbon group of a 5- or 6-membered ring formed by linkage with each other.
Moreover, particularly preferred is a monomer represented by the formula (1) shown above in which both R1 and R2are alkyl groups having 4 or less carbon atoms, one of R1 and R2is a hydrogen atom and the other is an alkyl group having 4 or less carbon atoms, or R1 and R2 are a hydrocarbon group of a 5- or 6-membered ring formed by linkage with each other, because such monomer is superior in copolymerizability with other monomers.
From the viewpoint of ease of production, a maleimide (meth)acrylate represented by the following formula (2) is more preferable. Furthermore, a tetrahydrophthalimide (meth)acrylic ester monomer represented by the following formula (3) is preferable because the resulting coatings have a good balance between hardness and processability. 
wherein R1 and R2 are independently a hydrogen atom or an alkyl group having 4 or less carbon atoms; one of R1 and R2 is a hydrogen atom and the other is an alkyl group having 4 or less carbon atoms; or R1 and R2 is a hydrocarbon group of a 5- or 6-membered ring formed by linkage with each other; R3 is an alkylene group which may be branched; R4 is a hydrogen atom or a methyl group; and n is an integer of from 1 to 6. 
wherein R is a hydrogen atom or a methyl group.
The copolymer of the present invention may further comprise other copolymerizable monomer units so long as its properties are not adversely affected. Such monomers are classified into those having a functional group other than hydroxyl group, epoxy group and alkoxysilyl group (hereinafter, referred to as xe2x80x9cother functional groupsxe2x80x9d) and those having no functional group.
Examples of the monomers having other functional groups include carboxylic acid-containing monomers such as (meth)acrylic acid, itaconic acid, maleic acid, fumaric acid, vinyl acetate, and crotonic acid.
Examples of the monomers having no functional group include xcex1-olefins such as ethylene, propylene, and isobutylene; partially halogenated olefins such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; allyl compounds such as allyl acetate and allyl butyrate; crotonates such as ethyl crotonate and propyl crotonate; vinyl ethers such as ethyl vinyl ether and cyclohexyl vinyl ether; vinyl carboxylates such as vinyl acetate, vinyl propionate, and vinyl versate; styrenes such as styrene, xcex1-methylstyrene, and vinyl toluene; acrylonitrile; acrylamide; and methacrylamide. These monomers can be used either singly or in combination.
It is preferred that the copolymer of the present invention has a copolymerization ratio as follows:
constitutional unit (A): 20 to 95% by weight;
constitutional unit (B): 1 to 50% by weight;
constitutional unit (C): 1 to 40% by weight;
other constitutional units (D): 0 to 40% by weight.
If the proportion of the unit (A) is less than 20% by weight, solvent solubility and film-forming properties of the coating are degraded, and if the proportion exceeds 95% by weight, curability is deteriorated. If the proportion of the unit (B) is less than 1% by weight, curability is degraded, and if the proportion exceeds 50% by weight, processability for coating is deteriorated. If the proportion of the unit (C) is less than 1% by weight, coating gloss and weatherability are not improved, and if the proportion exceeds 40% by weight, stability of the paint is deteriorated. Furthermore, although other constitutional units (D) can be used in such amounts as not to adversely affect the properties of the polymer, a proportion thereof exceeding 40% by weight makes the paint less stable.
Particularly preferred proportion of each constitutional unit is:
constitutional unit (A): 50 to 80% by weight;
constitutional unit (B): 5 to 35% by weight;
constitutional unit (C): 5 to 25% by weight;
other constitutional units (D): 0 to 30% by weight.
It is preferred that a (meth)acrylic ester unit having an alkyl group containing 8 to 20 carbon atoms accounts for 5 to 50% by weight of the total amount of the constitutional monomer units of the copolymer, because lifting of paint coatings formed from the resulting paint composition hardly occurs. The term xe2x80x9cliftingxe2x80x9d refers to the phenomenon of swelling or peeling of old paint coatings, which occurs when new paints are coated thereon, and such phenomenon causes the resulting coating to be defective. When the amount of the (meth)acrylic ester unit having an alkyl group containing 8 to 20 carbon atoms exceeds 50% by weight, the resulting coating becomes much softer, which is sometimes unfavorable.
The average molecular weight of the copolymer is, in terms of number average molecular weight as determined by gel permeation chromatography using polystyrene as a standard, preferably 1,000 to 1,000,000. In the case where the copolymer is used as a paint polymer which is soluble in an organic solvent, it is particularly preferred that its number average molecular weight is 3,000 to 30,000. When the copolymer is used in the form of an emulsion or suspension in an aqueous medium, it is particularly preferred that its average molecular weight is 10,000 to 500,000. When the polymer is used in a powder paint, it is particularly preferred that its average molecular weight is 3,000 to 50,000.
It is suitable that the glass transition temperature of the copolymer (hereinafter, abbreviated as Tg) is within a range of 5xc2x0 C. to 100xc2x0 C., particularly 5 to 80xc2x0 C. If the Tg is lower than 5xc2x0 C., stain resistance of the coating degrades, and if the Tg exceeds 80xc2x0 C., processability for coating is deteriorated. Moreover, if the Tg exceeds 100xc2x0 C., smoothness of the heat-cured coating is degraged. For use as a powder paint, it is preferred that the copolymer has a Tg of 35xc2x0 C. or higher. If the Tg is lower than 35xc2x0 C., when powdered, possibility of blocking arises.
The above-mentioned copolymer can be prepared by radical-polymerizing the above-described monomers from (A) to (D) in the presence of a radical polymerization initiator. As the polymerization method, bulk polymerization, suspension or emulsion polymerization in an aqueous medium, solution polymerization in an organic solvent, and others are adoptable.
As the radical polymerization initiator, peroxides such as diisopropyl peroxydicarbonate, tert-butylperoxypivalate, benzoyl peroxide, and lauroyl peroxide, azo compounds such as azobisisobutyronitrile and azobisisovaleronitrile, and inorganic peroxides such as ammonium persulfate and potassium persulfate are available. It is preferred that the initiator is used in an amount of from 0.0001 to 10% by weight relative to the total amounts of the monomers.
As an emulsifier for use in emulsion polymerization, use of an emulsifier of the anion- or nonion-type, and their concomitant use are preferred. Examples of the emulsifier include higher alcohol sodium sulfates and polyethylene glycol alkylethers, etc. As a reactive emulsifier, xe2x80x9cLumtelxe2x80x9d (trade name, manufactured by Kao Corp.), xe2x80x9cEleminolxe2x80x9d (trade name, manufactured by Sanyo Chemical Industries Ltd.), xe2x80x9cAquaronxe2x80x9d (trade name, manufactured by Dai-ichi Kogyo Seiyaku, Co., Ltd.), or xe2x80x9cAdekariasoapxe2x80x9d (trade name, manufactured by Asahi Denaka Kogyo, K. K.) may be used. The amount of the emulsifier is preferably within the range of from 0.1 to 50 parts by weight relative to a total of 100 parts by weight of the monomers.
Examples of the organic solvent which is used in solution polymerization include cyclic ethers such as tetrahydrofuran and dioxanes; hydrocarbons such as n-hexane and cyclohexane; aromatic hydrocarbon compounds such as benzene, toluene, and xylene; esters such as ethyl acetate and butyl acetate; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; alcohols such as ethanol, isopropanol, n-butanol, and n-butyl cellosolve; and orthoacid esters such as trimethyl orthoformate and trimethyl orthoacetate. These can be used either alone or in combination. It is preferred that the amount of the organic solvent to be used is within a range of from 20 to 200 parts by weight relative to 100 parts by weight of the total of the monomers. If necessary, a chain transfer agent such as laurylmercaptane may be added.
If the paint composition is one containing an organic solvent, for its efficient production, solution polymerization in which the organic solvent is utilized as a reaction solvent is preferred.
Although no particular restrictions are placed on the polymerization conditions, the polymerization temperature is preferably from 20 to 140xc2x0 C., and the polymerization is usually effected under ordinary pressure. The polymerization time is preferably from 3 to 40 hours. All the monomers to be polymerized may be batched in the initial stage of the reaction, or portions of the monomers may gradually be added as the polymerization proceeds. If necessary, as a pH adjusting agent, potassium carbonate, sodium hydrogencarbonate, hydrotalcite, an anion-exchanged resin, or others may be added.
The paint composition of the present invention is produced by mixing the above-described copolymer with, if necessary, a variety of additives generally incorporated into paints.
Preferably, the paint composition is one further containing an organic solvent as an essential component, because such composition shows better film-forming properties and coatings formed therefrom tend to present better gloss. Specific examples of the organic solvent include cyclic ethers such as tetrahydrofuran and dioxanes; aromatic hydrocarbons such as benzene, toluene, and xylene; esters such as ethyl acetate and butyl acetate; ketones such as methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; saturated hydrocarbons such as n-hexane, cyclohexane, methylcyclohexane, ethylcyclohexane, heptane, octane, nonane, decane, undecane, dodecane, and tridecane; mineral spirits such as xe2x80x9cNaphthaxe2x80x9d No.5 and No.6 (trade name, manufactured by Exxon Chemical Co.), and xe2x80x9cLAWSxe2x80x9d and xe2x80x9cHAWSxe2x80x9d (trade name, manufactured by Shell Chemical Company); high boiling point aromatic compounds such as xe2x80x9cSorbesso 100xe2x80x9d and xe2x80x9cSorbesso 150xe2x80x9d (trade name, manufactured by Exxon Chemical Co.); and orthoacid esters such as trimethyl orthoformate and trimethyl orthoacetate.
An organic solvent selected from the group consisting of cyclic ethers, aromatic hydrocarbons, esters, and ketones makes the resulting paint composition exceptionally good in storage stability and high in solid matter content, and thus, is favorable.
An organic solvent containing 40% by weight or more of a saturated hydrocarbon makes the resulting paint composition unsusceptible to the lifting phenomenon, and thus, is favorable. An organic solvent containing 50% by weight or more of a saturated hydrocarbon is more preferred.
Moreover, it is preferred that an organic solvent having a boiling point of 60xc2x0 C. or higher is used. In the case where the constitutional unit (B) is a monomer unit having an epoxy group, use of an organic solvent having a boiling point higher than 80xc2x0 C. is more preferred. An organic solvent of which the boiling point is lower than 60xc2x0 C., when the paint composition is made into a paint, may let the paint dry too quickly, resulting in poor film forming properties. It is preferred that the ratio of the copolymer to the organic solvent is from 25:75 to 75:25 (weight ratio).
Further, the copolymer may be used in the form of an aqueous paint by emulsifying with or microsuspending in an aqueous medium, in which case a water-soluble organic solvent such as ethanol, ethylene glycol, butyl cellosolve, xe2x80x9cSolfit Acetatexe2x80x9d (trade name, manufactured by Kuraray Co., Ltd.), or xe2x80x9cTexanol CS12xe2x80x9d (trade name, manufactured by Chisso Corp.) may be added thereto and it is preferred that the ratio of the copolymer to the aqueous solvent is 30:70 to 60:40 (weight ratio).
If the constitutional unit (B) of the copolymer of the present invention comprises a monomer unit having a hydroxyalkyl group, the paint composition may further comprise a curing agent which is reactive with hydroxyl group. Such paint composition provides coatings particularly excellent in hardness, solvent resistance, and durability, and therefore, is favorable. As the curing agent reactive with hydroxyl group, there may be mentioned polyisocyanate compounds and aminoplast compounds.
Examples of the polyisocyanate compounds include diisocyanate compounds such as aliphatic diisocyanates typified by tetramethylene diisocyanate, trimethylhexamethylene diisocyanate, and hexamethylene diisocyanate; aromatic diisocyanates typified by tolylene diisocyanate, diphenylmethane diisocyanate, and xylylene diisocyanate; and alicyclic diisocyanates typified by isophorone diisocyanate, 4-4xe2x80x2-methylenebis(cyclohexylisocyanate), and methylcyclohexane-2,4-diisocyanate; dimers and trimers thereof; and adducts and biuret thereof that can be obtained by reacting these diisocyanate compounds with water or polyhydric alcohols; including block isocyanates in which isocyanate groups are protected by other groups.
Specific examples of the aminoplast compounds include melamines such as methylated melamine and butylated melamine; urea resin; and benzoguanamine.
Addition amount of the curing agent reactive with hydroxyl group is, in the case of it being a polyisocyanate compound, in such an amount that the molar ratio of the NCO group of the curing agent to the hydroxyl group of the hydroxyalkyl group-containing copolymer (NCO/OH) is within a range of from 0.2/1.0 to 2.0/1.0, more preferably 0.5/1.0 to 1.5/1.0. If the curing agent employed is an aminoplast compound, it is preferred that the agent is used in such an amount that the weight ratio of the hydroxyalkyl group-containing copolymer to the curing agent (copolymer/curing agent) is within a range of from 3/1 to 10/1. The curing agent may be used together with a curing accelerator such as dibutyl tin dilaurate or p-toluenesulfonic acid.
When the constitutional unit (B) of the copolymer of the present invention comprises a monomer unit having a hydroxyalkyl group, the paint composition can further comprise a hydrolyzed condensate of an alkoxysilane, in which case coatings formed therefrom are excellent in stain resistance and thus preferable. The hydrolyzed condensate of an alkoxysilane is a hydrolyzed condensate of an alkoxysilane having two or more hydrolyzable alkoxy groups within one molecule, and one having a degree of condensation of about 2 to 10 is preferable in that such a condensate shows good compatibility with the copolymer of the present invention and has an excellent ability of improving the stain resistance. As such a compound, those commercially available from Colcoat Co., Ltd. under the trade names, for example, xe2x80x9cES40xe2x80x9d (condensate of approximately pentamer of tetraethoxysilane on an average) and xe2x80x9cMS51xe2x80x9d (condensates of approximately tetramer of tetramethoxysilane on an average) can be used. In addition, those synthesized by the processes recited in Japanese Laid-Open (Kokai) Patent Publication No. Hei 8-176304, Japanese Laid-Open (Kokai) Patent Publication No. Hei 8-113755, and Japanese Laid-Open (Kokai) Patent Publication No. Hei 9-31399 can also be employed. The amount of the hydrolyzed condensate of an alkoxysilane formulated into the paint composition is, in terms of solid matter content of the condensate in a solution, 2 to 100 parts by weight, preferably 5 to 80 parts by weight per 100 parts by weight of the copolymer. If the proportion of the alkoxysilane condensate is less than 2 parts by weight, stain resistance is not improved, but a proportion exceeding 100 parts by weight sometimes makes the coating brittle.
If the constitutional unit (B) of the copolymer of the present invention is one comprising a monomer unit having an epoxy group, it is also possible to use the copolymer in a form of a powder paint by isolating the polymer from monomer residues and volatile matters such as the organic solvent. In the steps of removing residual monomers and the solvent, there can be adopted a technique of re-precipitation in a poor solvent of the copolymer obtained by polymerization in an aqueous medium or an organic solution followed by drying, a technique of thin film hardening by drying with a centrifugal-film evaporator or a conveyor dryer, or a technique of spray-drying. It is preferred that the solid copolymer thus obtained has a volatile matter content of 1% or less. It is also possible to purify bulk-polymerized copolymers by heat-drying under reduced pressure.
When the polymer is used as a powder paint, the polymer is, after having been milled by an impact hammer mill, a vibrating mill, or a Willey grinding mill, mixed with other components, in which case the particle size of the polymer is preferably about 10 to 250 xcexcm.
If the constitutional unit (B) of the copolymer of the present invention is one comprising a monomer unit having an epoxy group, it is preferred that the paint composition is used together with a curing agent reactive with an epoxy group. Examples of such curing agent are acid group-containing compounds and amino group-containing compounds.
Examples of the acid group-containing compounds include low molecular weight polyfunctional carboxylic acids such as succinic acid, phthalic acid, cyclohexanedicarboxylic acid, and dodecanedioic acid; homopolymers of (meth)acrylic acid; copolymers of (meth)acrylic acid with other monomers; polymers obtained by reacting hydroxyl group-containing polymers with acid anhydrides to introduce a carboxylic acid group thereinto; and sulfonic acid group-containing polymers. Examples of the amino group-containing compounds include low molecular weight amino compounds such as tetraethylenediamine, hexamethylenediamine, hexamethylenetetramine, and dicyandiamide; and polymers containing an amino group. Preferred as the curing agent is, from the viewpoint of the water resistance of the resulting coating, a compound containing an acid group. It is preferred that the curing agent is used in such an amount that the ratio of the functional group in the curing agent is 0.1 to 2.0 equivalents per 1 equivalent of the epoxy group. Together with the curing agent described above, a curing accelerator such as dibutyl tin dilaurate and p-toluenesulfonic acid may be added to the paint composition.
If the constitutional unit (B) of the copolymer of the present invention is one comprising a monomer unit having an alkoxysilyl group, the paint composition, after forming a coating, readily starts reaction upon exposure to atmospheric moisture and cures to form a tough hard coating. In this case, for accelerating the curing rate, addition of a curing accelerator is desirable. Examples of the curing accelerator include organic metals such as cobalt naphthenate, lead naphthenate, calcium octylate, lead octylate, zinc octylate, and cobalt octylate; organic titanium compounds such as tetrapropyl titanate and tetrabutyl titanate; and organic tin compounds such as dibutyl tin dilaurate, dioctyl tin dilaurate, dibutyldichlorotin, dibutyl tin diacetoacetate, and triethyl tin monostearate. Addition amount of the curing accelerators is preferably from 1 ppm to 5% by weight, more preferably from 10 ppm to 1% by weight relative to the copolymer.
If necessary, to the paint composition of the present invention may added a pigment, a metal powder, an ultraviolet ray absorber, a light stabilizer, a fluidity adjusting agent, a leveling agent, a slip agent, a dispersing agent, a color separation inhibitor, an antioxidant, a foam inhibitor, a flame retardant, a thickener, a surface condition regulating agent, a silane coupling agent, and others.
Examples of the pigment are inorganic pigments such as titanium oxide, iron oxide, red iron oxide, calcined pigments, and pearl pigments; organic pigments such as phthalocyanine blue, benzidine yellow, quinacridone, quinacridone red, isoindolinone, and carbon black; loading pigments such as calcium carbonate and barium sulfate; and metallic pigments such as aluminum flakes and stainless flakes. Examples of the metal powder are stainless, aluminum, and bronze powders. Moreover, as a frosting agent, polyethylene wax, polypropylene wax, or a silica-based frosting agent may be added. As the ultraviolet ray absorber, for example, an organic ultraviolet ray absorber typified by a benzophenone-series compound, a benzotriazole-series compound, and an oxalic anilide-series compound, or an inorganic ultraviolet ray absorber typified by a microparticulate titanium oxide and cerium oxide can be used. As the silane coupling agent, in addition to xcex3-glycidoxypropyltrimethoxysilane, xcex2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, xcex3-isocyanatepropyltriethoxysilane, and xcex3-mercaptopropyltrimethoxysilane, those commonly employed as additives for paint compositions can be used. In the case where the paint composition is dissolved in an organic solvent, to the solvent may be added a dehydrating agent such as orthoformates, orthoacetates, and tetraalkoxysilanes.
The paint composition of the present invention can be applied to such a substrate as a steel plate, stainless, aluminum, concrete, mortar, plastic, and wood with use of a spray, a brush, a roller, or a bar coater.
If the constitutional unit (B) of the copolymer of the present invention is one comprising a monomer unit having an epoxy group, flow immersion, electrostatic coating, or other coating methods are also adoptable. In this case, the paint composition applied is then heated at a temperature of from 130 to 200xc2x0 C. in an infrared furnace, a dielectric heating furnace or an air heating furnace, or by other heating means, so as to cure to form a hard coating.
The paint composition of the present invention is also available for use in mending old coatings. Specific examples of old coatings that can be mended, i.e., undercoats, include those made of thermosetting or thermomelting paints such as acrylic resin paints, alkyd resin paints, urethane resin paints, epoxy resin paints, acrylic silicone paints, vinyl resin paints, phenol resin paints, polyester resin paints, fluororesin paints, and melamine resin paints; and paints that dry at ordinary temperature such as acrylic resin paints, alkyd resin paints, urethane resin paints, epoxy resin paints, chlorinated rubber paints, fibrin derivative paints, fluororesin paints, oil paints, and vinyl resin paints.
Preferred paint compositions for use in mending coatings are those containing an organic solvent. Of these, paint compositions which contain an organic solvent having a saturated hydrocarbon content of 40% by weight or more, particularly 50% by weight or more, are preferred because such paint compositions are unsusceptible to the lifting phenomenon and thus provide coatings with a good finish.
Also preferred paint compositions for use in mending coatings are, due to their unsusceptible property to the lifting phenomenon, those in which a (meth)acrylic ester unit having an alkyl group of 8 to 20 carbon atoms accounts for 5 to 50% by weight of the total of the constitutional units of the copolymer of the present invention.
In the repair method of an old coating, if the old coating is bulging, has chalked, or has been stained with dirty substances, it would be desired that the paint composition of the present invention is applied after the old coating has been abraded with sand paper or a brush or washed with high pressure water. Application of the paint composition can be conducted using a brush, a spray, a roller, or other tools. The paint composition of the present invention can also be applied to an old coating that has been pre-treated. Alternatively, the paint composition of the present invention may be applied after a paint made from a certain kind of resin selected according to the type of paints of the old coating has been coated on the old coating as a primer and dried. In this case, it is preferred that the primer and the paint composition of the present invention are applied at an interval of from half a day to 1 week. The film thickness of the paint composition of the present invention and that of the primer are preferably from 1 to 100 xcexcm independently.
Hereinafter, the present invention will be described concretely with reference to examples. In the following EXAMPLES, xe2x80x9cpart(s)xe2x80x9d means xe2x80x9cpart(s) by weightxe2x80x9d.